Meiosis is a special type of cell division that produces haploid gametes for sexual reproduction. During meiosis, chromosome pairing, synapsis and crossing over rely on homology between the paternal and maternal homologous chromosomes to ensure proper segregation and formation of gametes with the correct number of chromosomes. Crossovers are initiated by the formation and repair of induced double-strand breaks (DSBs) by homologous recombination. Defects in chromosome pairing disrupt DSB repair and result in checkpoint activation, leading to either apoptosis or the formation of aneuploid gametes. In males, sex chromosomes are largely hemizygous (i.e., lack a homologous chromosome), which presents a special challenge to repair DSBs and, moreover, to evade checkpoint activation. Our preliminary results show that DSB repair and checkpoint suppression occur in the context of a specialized chromatin structure found only on the X chromosome of males. Our overall hypothesis is that the epigenetic landscape of sex chromosomes and other aspects of male meiosis alter interactions with DSB repair and checkpoint machinery to ensure accurate transmission of the male genome through meiosis. To test this hypothesis we propose to elucidate the conserved mechanisms underlying chromosome behavior during male meiosis using the metazoan animal model Caenorhabditis elegans, which is particularly amenable to genetic, cell biological and molecular approaches. In Aim 1 we will define the pathways that mediate DSB repair on hemizygous regions of sex chromosomes by analyzing repair of both meiotic and engineered DSBs cytologically, molecularly and functionally in wild type and repair-defective mutants. In Aim 2 we will elucidate the role of specific histone modifications on DSB repair and checkpoint silencing by analyzing the recruitment of DSB processing factors and checkpoint proteins to DSBs in germ lines with altered chromatin. Here we will also probe the physical interactions between chromatin, chromatin modifiers, DNA repair and checkpoint proteins. In Aim 3 we will define global differences in DSB processing in the male versus female germ line. Together, an understanding of these processes in this genetically tractable system will provide novel and important insights into how the meiotic program is modified to promote successful male meiosis. These studies have direct relevance to understanding the increased frequency of sex chromosome aneuploidy associated with human meiosis resulting in developmental disorders including Turner and Klinefelter's Syndromes. Importantly, these studies will also elucidate general mechanisms of DNA repair and checkpoint signaling, which play critical roles in monitoring and maintaining genome integrity in all cells.